About 30,000 people live in the potential path of the mudflow in the Puyallup River Valley, though no major cities would be in danger.
The U.S. Geological Survey scientists could not predict when the 14,410-foot volcano might erupt -- the last time was 150 years ago, and the last mudflow was about 500 years ago -- but they employed a new technique to identify sections of weakened rock that might give way in an eruption.
The researchers, who reported their findings in Thursday's issue of the journal Nature, took electrical and magnetic measurements by helicopter of the mountain and pinpointed 400 million tons of rock on the western side that have been weakened by heat and steam.
The technique will probably now be used to help identify towns in danger around other active volcanoes.
"It sounds like a wonderful technique," said Richard Fiske, a Smithsonian Institution geologist who has studied Rainier. "Any time you can make measurements without going over those steep, icy slopes, it's an advantage."
Big chunks of a mountain sometimes give way in volcanic eruptions, letting loose enormous mudflows sometimes swelled by lake water or melted snow. Known to geologists as lahars, these landslides can race downward at 45 mph or more like a 100-foot-deep river of concrete, plowing away or burying just about anything in their path.
Lahars killed more than 23,000 people when Colombia's Nevado del Ruiz volcano erupted in 1985. About 5,600 years ago, a huge Rainier mudflow drained all the way into Puget Sound, about 50 miles away.
In recent years, instruments have been placed on Rainier to measure the shaking of the ground from a large mudflow and give an early warning for evacuation.
But Steve Bailey, who oversees emergency plans for Pierce County to the west of Rainier, said: "I'm not certain I'm comfortable we can successfully evacuate the entire valley."
By JEFF DONN
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